With a conventional electrophotographic printer, a charging device charges the surface of a photoconductive drum or belt and an exposing unit such as an LED array head writes an electrostatic latent image on the charged surface of the photoconductive drum. Alternatively, arrays of microlasers, such as VCSELs may be used. U.S. Pat. No. 5,940,113 describes a full page VCSEL array for use in a printer.
As Shown in FIG. 1, the LED array head 100 comprises a wiring substrate 102 and driver IC's 103 mounted on this wiring substrate. The Driver IC's 103 drive LED array chips 104. The LED array chips emit light through a focusing rod lens array 101 to provide a LED array image 105, and to illuminate the charged surface in accordance with print data. The electrostatic latent image is developed with toner into a toner image, which is subsequently transferred to a print medium. The toner image on the print medium is then fixed by a fixing unit.
In the electrophotographic art, multi-colour printers are known to produce a plurality of colour toner images on a photoconductive drum or endless belt. The obtained toner images are transferred directly onto printing stock material such as a paper sheet or paper web material. In an alternative embodiment the toner images formed on a photoconductive recording member are transferred subsequently to an intermediate belt from distinct image forming stations and are then transferred simultaneously to a receiving sheet or web that eventually is cut into sheets containing a desired printing frame dimension.
The image quality that can be achieved with LED array heads is dependent on a number of factors including the resolution capability, which is determined by the addressability and the optical performance of the focussing rod lens array.
The addressability is typically expressed in terms of a number of pixel elements per unit of length or “dpi” (dots per inch). Along the direction in which the light-emitting elements are aligned on the print head (the X-direction), the addressability number results directly from the packing density of the Light Emitting regions on the LED array chips. The nature of the image data, the timing clocks and the specifics of the LED array head controller may allow to select an imaging mode in which the addressability in the perpendicular Y-direction differs from the addressability in the X direction. The resolution is further determined by the optical performance of the imaging optics that projects a 1:1 image of the Light emitting regions onto the photoconductive drum or belt. Poor optical performance typically results in oversized pixel images that overlap with the neighbouring pixels.
A second important factor determining the image quality level that can be achieved with LED array heads is the presence and the effectiveness of means to control the energy output of the individual pixel elements. Energy output control is required to compensate for inevitable small differences in energy output at the photosensitive medium (including effects of the 1:1 image lens).
Uniformity correction is typically achieved through the use of uniformity look-up tables that adjust the current to the LED's or the pulse length or a combination of both. Various methods for this uniformity control have been described and a preferred method for uniformity control may be found in EP0629974.
High quality systems equipped with uniformity compensation means are suitable for use with image controllers that provide multiple bit image data for each pixel which are then converted to selectable output energy levels for each pixel element.
It is known that the efficiency, i.e. the brightness of the light emission, decreases as the temperature of the LEDs increases. Also the lifetime of the LEDs drops with rising temperature. The decrease in brightness affects the imaging quality of an LED array exposure device. It is desirable to operate LED exposure devices at a temperature not surpassing 40° C., optionally at a temperature in the range of 250 to 35° C. For reliable pixel-wise imaging it is necessary that there is no substantial temperature difference or temperature gradient between the individual LEDs, as is e.g. in U.S. Pat. No. 5,177,500.
In the absence of effective means to reduce the temperature differences or gradients across the width of the print on the other hand, certain groups of pixels of the LED array may heat more than others as a result of recurring image data from repetitive print jobs, such as mailings with a given page lay-out. U.S. Pat. No. 5,177,500 describes the use of thermal shunts to reduce temperature gradients due to these effects.
According to one method to reduce effects induced by non-uniform heating uses LED arrays mounted on a thermally conductive carrier bar connected to a heat sink. The heat sink may comprise a metal panel provided with metal fins and for more extensive cooling, e.g. forced air-circulation, can be added EP 0629508 describes an improved LED array head where the heat sink is provided. The duct extends between a fluid inlet and a fluid outlet and has a cooling fluid such as water flowing there through. This cooling fluid can be temperature controlled by heating and cooling devices along the circulation path as to achieve a preset temperature for that cooling fluid.
Rather than providing bulky temperature control systems, compact lightweight LED array printer designs typically attempt to limit the heat generation. Improving the efficiency of the LED helps in that respect. A change from AlGaP technology to solid phase diffusion AlGaAs technology was reported to decrease LED-heating substantially in Proceedings of IS&T NIP14 p 405-408 (1998). Heat generation can additionally be reduced—at the expense of exposure power—by shortening pulse-times in matrix addressing techniques. Matrix addressing schemes with 8×8 wiring reduce the maximum duty cycle available by a factor 8. In the compact LED print heads for desktop printers the LED array chips are typically mounted on glass epoxy based circuit boards. Such boards are known to have very poor heat conductivity and there is no significant thermal conduction across the substrate in the sense of a heat sink as described above.
Digital colour printing using LED array technology started in 1994 with a 600 dpi LED array technology providing multi-level per pixel control up to 6 bit per pixel. These LED array technology based printing systems are used for digital printing in high capacity/high volume applications. Systems like the Xeikon 5000 digital printing press implement temperature control and comprise a metallic carrier structure thermoconductively coupled to a heat sink to flatten out temperature differences across the width of the print head
Existing 1200 dpi LED array head solutions including reported work on progress in this area fail at present to provide satisfactory solutions to control the temperature of the LED array head to the level required for high volume/high capacity applications as on a digital printing press.
One challenge for the design of a page wide LED array head of increased addressability is the precise positioning and alignment of the plurality of LED array chips on the wiring substrate. Each of said LED array chips has a plurality of light-emitting elements aligned and exposed on an upper surface thereof and consecutive LED array chips are to be aligned on said intermediate carrier bar in a direction in which said light-emitting elements are aligned with equal intervals.
FIG. 2 represent images from U.S. Pat. No. 6,559,879, that shows a typical configuration as known form prior art. LED array chips (9) and driver chips (10) are arranged on a wiring substrate (8), with wire bonds (11) for providing electrical current to each individual light emitting diode. In FIG. 2A driver chips (10) are provided on one side and in FIG. 2B driver chips (10) are provided on two sides of the row of LED array chips (9).
FIG. 3 shows the arrangement of two LED array chips 301 and 302 of a 600 dpi LED array head from OCE Printing Systems as published in “Handbook of Print Media” by H. Kipphan (Springer ISBN 3-540-67326-1 2001 p 694).
FIG. 4 schematically introduces positional parameters A and B into the arrangement as shown in FIG. 3. Distance (A) between the centre of the first or last light emitting area on the LED array chip and the LED array chip edge is usually about 16 microns. Gap B in between the LED array chips is usually about 10 micron. Bond pads for wire bonding are provided in an alternating fashion on either side of the LED array chip in a configuration with two drivers per LED array chip as in FIG. 2B. In this device, there are 128 light emitting elements per chip resulting in a chip length of about 5 mm.
U.S. Pat. No. 4,821,051 discusses aspects of thermal expansion differences between LEDs and the wiring substrate. The wiring substrate is comprised out of small tiles made out of steel.
U.S. Pat. No. 6,559,879 discusses thermal expansion and shrinking effects due to manufacturing steps and proposes the use of LED array chips bonded to a glass epoxy circuit board by an epoxy resin type soft adhesive.
The bonding of the LED array chips with a thin layer of a stiff adhesive is known to result in stress on the LED array chips as a result of a change in temperature when the CTE of the chips and the CTE of the wiring substrate differ. The use of a thicker layer of soft adhesive relieves such stresses. This has the effect however that shrinkage stress in the sense as discussed in U.S. Pat. No. 6,559,879 vanishes almost completely.
The gap B changes when changing temperature. The gap B, in absolute figures, is determined by the length of the chip and the difference CTE between the wiring substrate and the LED array chips.
The LED array chips 301 and 302 have light emitting regions 305. The PLF (pitch-last-first), which represents the distance between the centre of the rightmost light emitting region 306 of the left chip and centre of the leftmost light emitting region of the right chip is preferably exactly the same as the intended pitch P, measured between two adjacent LEDs 303 and 304 of the same LED array chip. In this embodiment, the Pitch P is 423 μm. in FIG. 3, further a bonding wire 310 is shown, and the location 311 where the light emitting area is bond.
For light emitting diodes at the ends of adjacent chips to be spaced at an interval matching the printing pitch, the chips have to be diced at positions very close to the light emitting cells, and no cracks or other defects can be allowed within the chip, (see OKI Technical Review April 2003/Issue 194 Vol. 70 No. 2—available from http://www.oki.com). Whereas the value A in FIG. 4 ideally would scale with the printing pitch (P), the ratio of A/P typically increases with decreasing printing pitch or increasing addressability. This increase of A/P affects the size of the gap B.
In the case of small gap B of only a few microns, even a moderate temperature decrease may lead to a temperature induced reduction of the gap B to zero leading to a collision between the LED array chips, especially when the length of the chip and the difference CTE between the wiring substrate and the LED array chips are substantial.
Whereas conventional Cu-based wiring substrates can be used at 600 dpi for a LED array chip size of 5 mm where light-emitting elements aligned with equal intervals have a pitch between consecutive light-emitting elements of 42.3 microns, the gap B is reduced to zero when subjected to only a limited temperature reduction. When the pitch between consecutive light-emitting elements is reduced below 35 microns, e.g. as in 800 dpi systems where reasonable LED array chip sizes of more than 4 mm require to pack at least 192 light emitting elements per chip, there is a risk in collision of adjacent LED array chips when the LED array head is subjected to too large temperature reduction, e.g. in case of transporting the LED array head or devices in which the head is integrated by airplane. As an example, such device may be subjected to temperatures of −30° C. during transport, which may damage the LED array heads because of collision of adjacent LED array chips.